Gaseous streams comprising methane and heavier hydrocarbons occur naturally, i.e., as natural gas, or are an important by-product of a variety of refinery processes. It is often desired to recover the methane from such streams for such use as a fuel or as a reactant. One method for recovering methane particularly suited for treatment of natural gas streams involves cooling the stream to condense the heavier hydrocarbons from which the methane can be separated by distillation. The distillation is usually accomplished on a stream precooled to at least about -100.degree. F.
A specific process for obtaining methane from natural gas involving distillation of methane from condensed heavier hydrocarbons is described in detail in U.S. Pat. No. 3,292,380, the disclosure of which is incorporated herein by reference. In that process, the gas stream under high pressure is first cooled in a first stage by indirect heat exchange to about 32.degree. F or below. The gas is then expanded through a turbine to cool it to below about -100.degree. F thereby obtaining a condensate comprising the heavier hydrocarbons. The methane in the stream is removed as overhead from a stripping column. The heavier hydrocarbons are recovered from the stripper and may require further processing to produce a liquified product (LPG). The LPG is subsequently cooled and transported to storage.
In this and related processes, the methane is recovered at too low a pressure for economical transportation by pipeline. As a result, it must be compressed before being discharged through the pipeline. As a result of this compression, typically to about 500 p.s.i.g. or greater, the methane is heated, usually to at least 170.degree. F and sometimes as high as 400.degree. F. In the processes known to the prior art, the heat content of the compressed gas is usually wasted as the temperature at which it is available is typically regarded as being too low for efficient recovery.
By contrast, the LPG product is usually obtained at a temperature higher than that at which storage is convenient. Therefore, prior to storage the LPG is cooled, preferably to about 45.degree. F or lower. In the past, cooling of the LPG has usually been accomplished by using conventional compression refrigeration means. Fuel for the refrigeration compressor has been obtained by diverting a portion of the methane gas obtained as product. Typically a gas engine or turbine is used to drive the compressor.
The recent escalation of fuel costs has made the prior art process less attractive inasmuch as it consumes a portion of the relatively expensive methane product. Therefore, it should be apparent from the foregoing that a process avoiding the consumption of a portion of the methane product would constitute a valuable advance in the art. Accordingly, one object of this invention is to provide an improved process for separating methane from heavier hydrocarbons by distillation.
Another object of this invention is to provide a process for separating methane and LPG fractions from a stream of gaseous hydrocarbons whereby it is not required to consume any of the methane produced as fuel for condensing and subcooling LPG produced in the process. Yet another object of this invention is to provide an improved process involving the distillation of methane from condensed heavier hydrocarbons whereby the heat content of a compressed methane gas product is efficiently recovered.
The manner is which these and other objects may be attained will be apparent from a consideration of the following description of the invention.